1. Field of the Invention
The present invention relates to a projection type display device comprising a not optical but electrical trapezoidal distortion correcting function.
2. Description of the Prior Art
FIG. 1 illustrates an example of the installation of a liquid crystal projector 1 in a case where the liquid crystal projector 1 is arranged with the top thereof directed upward toward a screen 2.
When the liquid crystal projector 1 is arranged with the top thereof directed upward toward the screen 2, a lower part of a video projected onto the screen 2 shrinks, as shown in FIG. 2a, thereby causing trapezoidal distortion.
When an upper part of the video is shrunken by signal processing (electrical trapezoidal distortion correction processing) with respect to such trapezoidal distortion that the lower part of the video shrinks, the contour of the video projected onto the screen 2 becomes square, as shown in FIG. 2b, thereby correcting the trapezoidal distortion.
FIG. 3 illustrates an example of the installation of the liquid crystal projector 1 in a case where the liquid crystal projector 1 is arranged with the top thereof directed downward toward the screen 2.
When the liquid crystal projector 1 is arranged with the top thereof directed downward toward the screen 2, an upper part of a video projected onto the screen 2 shrinks, as shown in FIG. 4a, thereby causing trapezoidal distortion.
When a lower part of the video is shrunken by signal processing with respect to such trapezoidal distortion that the upper part of the video shrinks, the contour of the video projected onto the screen 2 becomes square, as shown in FIG. 4b, thereby correcting the trapezoidal distortion.
When the trapezoidal distortion is corrected by the signal processing, however, an image must be compressed by being subjected to nonlinear filtering in both the horizontal and vertical directions, as already known. When the image is compressed in a case where the trapezoidal distortion is corrected, the amount of information related to the image decreases, resulting in deterioration of the image quality and omission of the amount of information.
Meanwhile, the video projected onto the screen 2 may include an OSD (On Screen Display) image. In many cases, used as the OSD image is one having important information related to a liquid crystal projector such as image control or function switching displayed thereon using a combination of characters, drawings and icons (pictures). Further, the OSD image is fine because its font or the like is fine in order to transmit more information in its character.
FIG. 5a illustrates a video projected onto the screen in a case where the liquid crystal projector 1 is arranged, as shown in FIG. 1, and an OSD image A is included in an upper part of the video. FIG. 5b illustrates a video projected onto the screen in a case where the video as shown in FIG. 5a is subjected to trapezoidal distortion correction.
Such trapezoidal distortion that a lower part of the video shrinks, as shown in FIG. 5a, is corrected by compressing an upper part of the video, similarly to the lower part thereof, as shown in FIG. 5b. However, a compression rate increases toward an upper part of the screen, and the degree of decrease or omission of the amount of information on the screen increases in proportion thereto. When the OSD image A is included in the upper part of a trapezoidal video whose lower part shrinks, as shown in FIG. 5a, therefore, the OSD image is difficult to see in the video which has been subjected to trapezoidal distortion correction, as shown in FIG. 5b. 
That is, a video signal other than the OSD image is similarly compressed and corrected. In the video signal, however, its field or frame constitutes one image. Even if a part of an upper part or a lower part of the video signal is compressed so that the amount of information decreases, therefore, it is possible to understand the contents of the image. Contrary to this, the OSD image does not perform its inherent function when the amount of information related to its part decreases so that characters are difficult to read, for example.